A new optical sensor for carbon dioxide has been developed. The sensor is based on the principle that pH of a bicarbonate solution in equilibrium with gaseous carbon dioxide can be estimated by measuring the light transmittance of an indicator dye. A flexible probe about 60 cm long with a sensitive tip was made using two plastic optical fibers. One carried light from a filtered incandescent lamp to the probe tip and the other returned light that had passed through the phenol red dyed buffer to a photosensor. The tip is about 0.6 mm dia. and 0.8 mm long. Tests show good linearity over the range from 20 to 100 mm Hg and a 95% response time of one minute.